This invention pertains to fuel cell systems and, in particular, to fuel cell systems which utilize steam.
Fuel cell systems often utilize high pressure steam for reforming reaction (fuel processing) and for use as process steam for waste heat utilization. In reforming reaction, the steam is combined with a hydrocarbon fuel and the combination applied to a reformer which provides at its output fuel process gas to be used in the fuel cell or fuel cell stack of the system.
U.S. Pat. No. 3,969,145 discloses one steam generating practice wherein use is made of the heated oxidant and fuel process gases passing through the system fuel cell. Metallic tubes carrying coolant water are situated internal to the cell stack and in heat exchanging relationship with the respective flows of fuel and oxidant gases. The water in the tubes is thereby heated to produce steam which is also simultaneously heated in the same manner. The steam is then removed from the tubes and used elsewhere in the system as, for example, in steam reforming reaction of the type described above.
It has also been proposed to use the exhausted oxidant gas of the fuel cell system itself for steam generation external to the cell. In this case, the exhausted oxidant gas and water are supplied to a heat exchanger with the resultant production of steam.
In both the above practices, increased fuel cell temperature is required to provide a desired amount of steam at increased pressures. This can be seen from the equation governing the ratio of generated steam to generating fuel cell gas which is as follows: ##EQU1## where .DELTA.H is the latent heat of steam
C.sub.p is the heat capacity of gas PA1 t.sub.o is the initial temperature of the gas PA1 t.sub.p is a temperature equal to the steam saturation temperature t.sub.s which increases with desired steam pressure plus a small differential t.sub.d referred to as the pinch point.
Assuming that the gas stream is at a temperature of 375.degree. F. and that steam at 105 psia is required (this corresponds to t.sub.s =332.degree. F. and .DELTA.H=885 Btu/lb) and further that a differential t.sub.d =20.degree. F. is used and C.sub.p =0.28 Btu/lb .degree.F., then the ratio Q is calculated as follows: ##EQU2##
For steam at a pressure above 105 psia, the value of t.sub.p is increased while the values of .DELTA.H and C.sub.p remain substantially the same. As a result, to obtain at the higher pressure the same quantity of steam as obtained at the 105 psia level, the value of the fuel gas temperature t.sub.o must be increased by the amount of the increase in the value t.sub.p. This of course requires an increase in fuel cell operating temperature.
At steam pressures of the order of 100 to 180 psi, which pressures are desirable for many fuel cell systems or for many industrial process steam applications, the required increase in fuel cell temperature over conventional temperatures is such as to measurably decrease fuel cell life. As a result, use of the aforesaid practices to provide steam at these high pressures is undesirable.
One possible alternative to providing the increased pressure steam without raising fuel cell temperature, would be to use a compressor. However, this alternative is undesirable because of cost and power requirement considerations.
It is therefore an object of the present invention to provide a fuel cell system having an improved capability for generating steam.
It is a further object of the present invention to provide a fuel cell system capable of providing a given amount of steam at increasing pressures and useable for process gas reforming without having to increase fuel cell temperature.